For converting Zimbra mailbox into PST format, you should use the
Email Backup Tool. It efficiently backup emails from Zimbra into PST, PDF, EML, EMLX, MSG,MBOX, etc. It also allows users to migrate data from 90+ email clients into 15+ file formats and email clients. The tool has various features, such as keeping folder hierarchy, deleting duplicate items, a date range filter, etc. The software is available for both Windows and Mac OS. It also offers a free demo version, which converts 50 emails per folder for free.